This project proposed to morphologically and biochemically define the mechanism of action of vitamin A acid in altering epidermal differentiation in embryonic skin, normal skin, and in benign and malignant lesions of skin. Chick metatarsal skin, when cultured in vitro, is stimulated to undergo mucous metaplasia by vitamin A acid. Ultrastructural analysis alone cannot determine whether a post-translational effect, a genomic effect, or an effect of Vitamin A acid on the mesenchyme is the primary mechanism involved in the inhibition of keratinization and induction of epidermal mucin biosynthesis by vitamin A acid.